1. Field
This invention relates generally to U-tube steam generators and, more particularly, to such generators that disperse feedwater into a downcomer between a wrapper and the steam generator shell.
2. Description of Related Art
A pressurized water nuclear reactor steam generator typically comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a divider plate that cooperates with the tube sheet and a channel head forming a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with the primary fluid outlet header. The steam generator secondary side comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber made up of the shell on the outside and the wrapper on the inside and a feedwater ring disposed above the U-like curvature end of the tube bundle.
The primary fluid having been heated by circulation through the reactor enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header and through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced into the steam generator secondary side, i.e., the side of the steam generator interfacing with the outside of the tube bundle above the tube sheet, through a feedwater nozzle which is connected to a feedwater ring inside the steam generator. In one embodiment, upon entering the steam generator, the feedwater mixes with water returning from moisture separators. This mixture, called the downcomer flow, is conducted down the annular chamber adjacent the shell until the tube sheet located at the bottom of the annular chamber causes the water to change direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the tube wrapper. While the water is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to water surrounding the tubes causing a portion of the water surrounding the tubes to be converted to steam. To differentiate the steam/water mixture from the single phase downcomer flow, the fluid flow surrounding the tubes is designated as the tube bundle flow. The steam then rises and is conducted through a number of moisture separators that separate entrained water from the steam and the steam vapor then exits the steam generator and is typically circulated through a turbine to generate electricity in a manner well known in the art.
The U-shaped heat exchange tubes of such steam generators are typically described as having a hot leg, which is directly in fluid communication with the primary fluid inlet header and a cold leg which is directly in fluid communication with the primary fluid outlet header. A number of these types of steam generators preheat the downcomer flow by passing the cooler portions of the downcomer flow by the cold legs of the tube bundle to increase the log mean temperature difference and thereby enhance heat transfer. This is accomplished by employing a partition plate which extends across the tube sheet through a center tube lane between the hot legs and cold legs of the heat exchange tubes. The partition plate extends axially up between the tubes from the tube sheet to an elevation below the U-bends. In this preheat class of steam generators, the downcomer region typically extends less than 180° around the cold legs side of the wrapper and is partitioned to separate the downcomer region from the circumferential area around the wrapper that surrounds the hot legs. A nearly semicircular feedwater distribution ring is supported above the cold leg downcomer region in the partitioned area between the shell and the wrapper, so that the feedwater is distributed down the outside of the wrapper surrounding the cold legs, underneath the wrapper at the tube sheet and up and around the cold legs of the heat exchange tubes.
Axial flow preheat steam generator feedrings must evenly distribute feedwater flow over approximately 160° of a steam generator's upper shell circumference. As explained above, this serves to introduce the colder feedwater into the cold leg side of the tube bundle, whereby the preheating benefit of increased heat transfer occurs. Prior art reference, U.S. Pat. No. 6,173,680, accomplished this objective utilizing a large inverted duct to direct and distribute flow into the downcomer, and included a loose parts screen within the feedring, which requires access through bolted flanges.
An improved feedwater distribution ring is desired that provides a much lower pressure drop loose parts screening arrangement with enhanced access features.
Furthermore, such a feedwater feedring design is desired that will accomplished substantially even feedwater distribution over the cold leg downcomer, using a more compact arrangement.